Personal rapid transit systems include small vehicles offering individual transport service on demand. This invention relates to automated vehicle systems such as personal rapid transit systems with vehicles travelling along tracks forming a network of stations, merges, and diverges interconnected by unidirectional links in the form of tracks. PRT vehicles may be constructed to be compact and light which in turn allows the PRT guide-way (track) structure to be light compared with conventional railroad systems such as conventional tramways or metro systems. Therefore, the construction cost of the PRT system is much lower than that of alternative solutions. A PRT system is more friendly to the environment, since it has less visual impact and generates low noise, and it does not produce local air pollution. Further, PRT stations can be constructed inside an existing building. On the other hand, since the headway/free distance may be kept comparably short, the traffic capacity of a PRT system is comparable with the existing traffic means such as bus and tramway.
Stations are normally located off-line on sidetracks so that stopping vehicles do not hinder passing vehicles.
The guideway/track network of a PRT system generally comprises unidirectional links/tracks and nodes (so-called merges or merge points) where two or more upstream tracks merge to form a downstream track as well as nodes (so-called diverges) where an upstream track divides to form two or more downstream tracks. An important issue for vehicles approaching diverges is the choice of route, while important issues for vehicles approaching a merge is safety, efficiency, and comfort. The present invention is concerned with safe and efficient merging strategies in PRT networks as well as in other networks where automated vehicles are travelling.
Generally, in a merge, two streams of vehicles come together and therefore a merge is a potential bottleneck for capacity. Whatever flow can pass through a merge can pass freely through the downstream network until the next merge. Merge capacity is thus dimensioning system capacity.
In typical automated vehicle systems no more than two streams of vehicles come together at a merge. However, it will be appreciated that the method described herein may also be applied in connection with merges where more than two vehicle streams merge.
Merges are also points of possible conflict in a PRT network and therefore safety critical. Normal methods for monitoring safe distance between vehicles are not sufficient for safety control in merges.
Generally a PRT system includes a speed control system for controlling speed and distance between vehicles. There are two main principles for vehicle control in PRT systems. With synchronous control vehicles are made to follow synchronously moving slots with constant time spacing, dimensioned to secure a safe distance at all permitted speeds in the network. Before a vehicle is allowed to depart from a station it is assigned a slot all the way to its destination. All bookings of merge passages need to be administered by a central computer. In a heavily loaded system, vehicles have to wait longer (taking up space) for a free slot, especially if its route passes through several merges. The usable capacity in a synchronous system is only about 65% of the theoretical link capacity. Regarding safety, as long as all vehicles follow their assigned slots there should be no merge conflicts.
With asynchronous control, merge conflicts are resolved locally as in car traffic. Vehicles can depart from a station as soon as there is a free slot on the main guideway but they may have to slow down or even stop before going through a merge. Traffic through a merge is controlled by a local merge controller independent of central control. Congestion can be reduced by dynamic routeing avoiding merges which tend to be overloaded. Merge capacities can be utilised up to 100% and vehicles can be dynamically rerouted if necessary. Thus, generally asynchronous control provides an improved system capacity, routeing flexibility and robustness towards disturbances.
U.S. 2004/0225421 describes a PRT system and a method of controlling movement of vehicles by means of a central control system, a wayside control system and a vehicle control system. When the wayside control system detects the identification of the approaching vehicle, the appropriate switch positions will be set and verified according to the traffic flow instruction from the central control system. However this document does not describe how to obtain a safe and smooth merge control.
DE 1.377.713 relates to vehicles moving freely in traffic systems, e.g. road traffic. The document describes a method of bringing together at an entry point on a single track, two converging streams of vehicular traffic, each stream moving along its individual lane. The movement of vehicles is based on inter-vehicle communication and always applies first-come-first-served. Since the method comprises manual operation by driver, driver support system and distance measuring onboard vehicle, the method is not readily applicable to PRT systems.
A traffic-light function as in car traffic may protect against merge conflicts in a PRT system, but would reduce capacity, since vehicles often would have to accelerate from standstill, thus creating longer inter-vehicle gaps than necessary and reduced speed through the merge.
It thus remains a problem to obtain a safe and smoother merge control in automated vehicle systems such as PRT systems. In particular, it is desirable to provide a merge control method and system that ensure safe distances between vehicles in merging vehicle flows while at the same time maintaining full capacity and smooth motion control for passenger comfort.